。
She's good at speaking English.
Literal
She [topic-は] English [object-を] speak [nominalizer-の] [subject-が] skilled is.
The nominalizer の turns the verb phrase 英語を話す ('speak English') into a noun-like unit, which is then marked by が as the thing she's good at. With 得意 (and the family 上手・下手・苦手), Japanese marks the activity with が, never を — even though it would be the verb's object outside of this construction. The formal copula である gives the sentence a slightly bookish, descriptive register; in conversation, ~のが得意だ or simply 上手 would be more natural.